Mechanistic Insights on the Reduction of CO₂ to Silylformates Catalyzed by Ir-NSiN Species

The hydrosilylation of CO₂ with different silanes such as HSiEt₃, HSiMe₂Ph, HSiMePh₂, HSiMe(OSiMe₃)₂, and HSi(OSiMe₃)₃ in the presence of catalytic ammounts of the iridium(III) complex [Ir(H)(CF₃CO₂)(NSiN*)(coe)] (1; NSiN*=fac-bis-(4-methylpyridine-2-yloxy); coe=cis-cyclooctene) has been comparatively studied. The activity of the hydrosilylation catalytic system based on 1 depends on the nature of the reducing agent, where HSiMe(OSiMe₃)₂ has proven to be the most active. The aforementioned reactions were found to be highly selective toward the formation of the corresponding silylformate. It has been found that using 1 as catalyst precursor above 328 K decreases the activity through a thermally competitive mechanistic pathway. Indeed, the reduction of the ancillary trifluoroacetate ligand to give the corresponding silylether CF₃CH₂OSiR₃ has been observed. Moreover, mechanistic studies for the 1-catalyzed CO₂-hydrosilylation reaction based on experimental and theoretical studies suggest that 1 prefers an inner-sphere mechanism for the CO₂ reduction, whereas the closely related [Ir(H)(CF₃SO₃)(NSiN)(coe)] catalyst, bearing a triflate instead of trifluoroacetate ligand, follows an outer-sphere mechanism.